Fructans are linear or branched polymers of repeating fructose residues with usually one terminal glucose unit. The number of residues contained in an individual polymer, also known as the degree of polymerization (DP), varies greatly depending on the source from which the polymer is isolated. Several bacteria can produce fructans with a DP 5000 or greater, while low DP fructans (DP 3 to 200) are found in over 40,000 plant species.
Based on their structure, several types of fructans can be identified in higher plants. The most characterized plant fructan is inulin. Inulin contains linear β(2-1)-linked fructosyl residues and commonly occurs in the Asterales such as Jerusalem artichoke (Helianthus tuberosus), sunflower (Helianthus sp.), Belgian endive (Cichorium intybus) and artichoke (Cynara scolymus). Inulin synthesis is initiated by sucrose:sucrose 1-fructosyltransferase (1-SST; EC 2.4.1.99) which catalyses the conversion of sucrose into isokestose (also named 1-kestose) and glucose. Additional fructosyl units are added onto isokestose, by the action of a fructan:fructan 1-fructosyltransferase (1-FFT, EC 2.4.1.100) resulting in a β(2-1)-linked fructose oligomer.
A second type of fructan is called levan and consists of linear β(2-6) linked fructosyl residues. Grasses such as Dactylis glomerata and Phleum pratense contain levans with a DP up to 200. Levans are synthesized by a sucrose:fructan 6-fructosyltransferase (6-SFT; EC 2.4.1.10) that uses sucrose as a fructosyl donor and acceptor to produce 6-kestose. Polymerization of 6-kestose is believed to be catalyzed by 6-SFT as well, using sucrose as the fructosyl donor.
A third type of fructan, graminan (also called mixed-levan), is found in many Poales such as barley and wheat. These plants use an SST to produce iso-kestose from sucrose, and 6-SFT to further polymerize isokestose, resulting in a fructan containing both the β(2-1) and the β(2-6) linked fructosyl residues.
The fourth type of fructan is often referred to as the neo-kestose series of fructans. The neo-kestose series have fructosyl residues on the carbon 1 and 6 of glucose producing a polymer with fructosyl residues on either end of the sucrose molecule. The inulin-neoseries found in Liliales such as onion (Allium cepa), leek (Allium porrum), and asparagus (Asparagus officinales) contain mainly a β(2-1)-linked fructose polymer linked to carbon 1 and 6 of glucose, while the levan-neoseries contain mainly a β(2-6)-linked fructose polymer linked to carbon 1 and 6 of glucose. Neoseries fructans are believed to be synthesized by the concerted action of 1-SST (producing isokestose) and 6G-FFT, a specific fructan:fructan 6G-fructosyltransferase that polymerizes fructosyl units onto carbon 6 of glucose.
Industrial applications of fructans are very diverse and range from medical, food, and feed applications, as well as the use of fructans as a raw material for the production of industrial polymers and high-fructose syrup. Regardless of size, fructose polymers are not metabolized by humans and animals. Fructans can enhance animal health and performance by being selectively fermented by beneficial organisms such as Bifidibacterium in the large intestine of animals, at the expense of pathogenic organisms such as E. coli and Salmonella, leading to altered fatty acid profiles, increased nutrient absorption, and decreased levels of blood cholesterol. Also, fructans have a sweet taste and are increasingly used as low-calorie sweeteners and as functional food ingredients.
Accordingly, there is a great deal of interest in understanding fructan biosynthetic pathways. With the isolation of nucleic acid fragments encoding various enzymes involved in the pathway, it may be possible to engineer transgenic plants to produce desired levels of different types of useful and novel fructans.